A known electronic engine control system comprises a processor-based engine controller that processes data from various sources to develop control data for controlling certain functions of the engine. Air and fuel management and recirculation of exhaust gas are examples of functions that are controlled by an engine control system. In a turbocharged diesel engine, the electronic engine control system also exercises control over turbocharger boost.
An important element of an emission control strategy in an engine involves the controlled recirculation of some exhaust gas to limit in-cylinder temperatures and hence limit the formation of NOx. Control of recirculated exhaust gas is performed by an exhaust gas recirculation (EGR) system that typically includes an EGR valve that is controlled by the engine control system. Variable valve actuation of engine cylinder valves may also be used for control of EGR.
An exhaust system of a diesel engine that comprises a DPF is capable of physically trapping diesel particulate matter (DPM) in engine exhaust gas passing through the exhaust system. This trapping of DPM prevents significant amounts of DPM from entering the atmosphere.
DPM includes soot or carbon, the soluble organic fraction (SOF), and ash (i.e. lube oil additives etc.). The trapping of those constituents by a DPF prevents what is sometimes seen as black smoke billowing from a vehicle's exhaust pipe. The organic constituents of trapped DPM, i.e. carbon and SOF, are oxidized within the DPF at appropriate times and under appropriate conditions to form CO2 and H2O, which can then pass through and exit the exhaust pipe to atmosphere. The ash collects within the DPF over time, progressively aging the DPF by gradually reducing its trapping efficiency.
A process commonly called regeneration counteracts increasing EBP by burning off accumulated soot in a DPF. Commonly owned U.S. patent application Ser. No. 10/217,729, filed 13 Aug. 2002, (Published Application No. US20040031262A1) describes both a natural regeneration process and a forced regeneration process. The latter process occurs whenever the level of accumulated soot becomes undesirably high and involves the use of the engine control system to raise exhaust gas temperature to a range that is high enough to burn off the accumulated soot. Natural regeneration may occur at any time during normal engine operation when temperatures that are sufficiently high to burn off soot occur in a DPF.
Factors that include a) possible natural regeneration at various times during normal engine operation when forced regeneration is not occurring, b) forced regeneration cycles when too much soot has been accumulated, and c) changing rate at which soot accumulates in a DPF due to aging, can cause variations in the manner in which a DPF performs as an engine operates. It appears that certain implications of these variations on other aspects of engine performance have not been fully appreciated.